USB (Universal Serial Bus) 2.0 is a data communication standard between a personal computer and a peripheral equipment. The USB 2.0 is capable of data transferring at 480 Mbps at the maximum as. In recent years, the USB 2.0 standard has been widely used.
According to the USB 2.0 standard, it is necessary to install a squelch circuit in a personal computer and a peripheral equipment. The squelch circuit judges whether receipt of a transferred data is to be approved or blocked according to a voltage difference between differential input voltages Vin1 and Vin2 received in the personal computer or the peripheral equipment.
When a voltage difference between differential input voltages Vin1 and Vin2 is above predetermined voltage Vhi, the squelch circuit is set non-active and outputs a ‘Low’ level signal to approve receipt of the transferred data. On the other hand, when a voltage difference between
differential input voltages Vin1 and Vin2 is below pre predetermined voltage Vlo, the squelch circuit is set active and outputs a ‘High’ level signal to block receipt of the transferred data. The relation between the predetermined voltages Vhi and Vlo is expressed as Vhi>Vlo.
However, when a voltage difference between differential input voltages Vin1 and Vin2 is larger than Vlo and less than Vhi, an output of the squelch circuit vibrates between the ‘High’ level and the ‘Low’ level to show a ‘Noisy’ level signal.
In order to evaluate operating characteristics of the squelch circuit, the voltage difference between the differential input voltages Vin1 and Vin2 is usually measured by sampling an output level of the squelch circuit to approve or block receipt of the transferred data, instead of measuring the differential input voltages Vin1 and Vin2
When the output of the squelch circuit is in the ‘Noisy’ state, the output may be judged as the ‘High’ level or the ‘Low’ level depending on timing of the sampling. Therefore, there is a problem that it is difficult to identify the ‘Noisy’ state of the output of the squelch circuit.
Further, when data receiving operation is controlled by such an output of the squelch circuit, a problem may occur that the data receiving state becomes unstable.
An improved squelch circuit is disclosed on Page 7 and in FIG. 6 of the Japanese Patent Application Publication (Kokai) No. 2005-354290. The publication shows that the squelch circuit is provided with a delay circuit to realize a filtering function to remove the ‘Noisy’ state, that is, a hazard.
However, the delay circuit described above determines the delay time by an analog operation of charging/discharging of a MOS capacitance formed in an LSI.
Thus, there is a problem that filtering characteristics of the delay circuit change due to fluctuation of characteristics in manufacturing the LSI.